87th AMS Annual Meeting

Monday, 15 January 2007
Water Balance of an Irrigated Watershed with Seasonally Drought in Southern China
Exhibit Hall C (Henry B. Gonzalez Convention Center)
Yuanshu Jing Sr., Nanjing Univ., Nanjing, China; and Z. Bin III and H. Zepp
The general objective of the study is to understand the regional hydroecological processes for land use planning against soil degradation. Different land-use systems and land use structures determines the different hydroecological processes and the influences on environment. Monitoring rainfall, soil water, surface runoff and discharge on different landscape scales will provide information on the hydrological components, their spatial and seasonal dynamics and their conversions. This information can be used to estimate run-on through the irrigated catchment, which is essential for calculating irrigation efficiency of extensive irrigation, and understanding of the comprehensive and combinative processes in space and time to non-monitored sites and land uses.

The study site is located in Yingtan, Jiangxi Province (2815' N, and 11655' E, elevation 47.5 m) south to the lower reaches of Yangtze River. It is a typical subtropical moist climate. Monthly rainfall decreased sharply after June, but rainstorms occasionally came with typhoons due to the monsoon climate. This climate favors multiple cropping in a year and causes the problems of seasonal drought and soil degradation due to soil and water erosion.

The small catchment was considered due to the least influence from the residential area and the on-farm-research in the watershed can be facilitated by cooperation with only three farmers. The catchment has well defined drainage divides and builds up a set of hydrologic subordinated entities. Each of them is clearly delineated by small channels and is completely covered by chestnut orchard, agroforestry, paddy fields and conventional peanut upland. The parent materials in the catchment include Quaternary red clay and different sandstones. These land uses and soil parent materials are typical in the area and the results derived from the catchment study can be easily extrapolated to the regional scale.

The land uses in the catchments are complicated. The valleys have been cultivated for rice cropping for a long history. The low hills were cultivated from indeciduous forest or Pine forest (Pinus massoniana) with sparse grass coverage into tea trees on contour in 1950s. The tea plantation was replaced by peanut (Arachis hypogaea) or fruit tree such as citrus trees (Citrus unshiu) and chestnut tress (Castanea mollissima) in 1980s, when a new land tenant system was implemented. The land tenant system allows farmers to have a long-term tenure of the state-owned land and make their own decisions on land use. Within an area of 46 ha, the typical land use systems were considered in the study region. The land use system includes forest system of chestnut at a spacing of 4 m 4 m, with main canopy around 4-6 m above the ground, established in 1996, agroforestry system of citrus, with main canopy around 3-4 m above the ground, at a spacing 4 m 4 m intercropped with peanut established in 1998, annual crop peanut at a spacing 20 cm and paddy rice. Crop and forest development stage is shown in Table 3. Tensiometers under each land use system were installed at different depth at different positions of slope in the winter of 2000. The tensiometers plots were fenced in a square shape to avoid any potential damage due to farm tillage and feeding cattle.

Soil water potential was recorded in two ways. The first way was to read the tensiometers with readout manually at 9:00 am every two days. The second way was to automatically record the data every 10 minutes using data logger (DELTA-T Devices, Dl2e, Cambridge, U.K.) via T3 transducers that were connected to tensiometers. There are occasions of missing data because of various environmental problems, including frozen tensiometers, severe summer drought and invaded water inside the small hub box. The tensiometers, which were manually read, were collected to supplement the missing data after the calibration between the two methods. Daily precipitation at the catchment was collected by 20-cm diameter rain gauge, connecting to the data logger.

This study shows how data obtained at plot experiments may be extrapolated to the small catchment scale and combined with other hydrological variables for catchment balance and irrigation performance evaluation, using such spatial data sets as land cover, soils, and catena. The total water inputs at small catchment during monitoring periods were irrigation 714.0 mm and rainfall 4326.8 mm, whereas the water outputs were surface discharge 10692.6 mm and evapotranspiration (ET) 1943.6 mm. The actual evapotranspiration was 20.6% lower than the climatic evapotranspiration, showing that the plants at the small catchment were water-stressed. ET deficit in the dry year was 3 times as the normal year. The total ET estimates were highest for peanut (1080.5 mm), which covered 49.7% of the small catchment. The ET estimate of other land use was 203.2 mm for agroforest of citrus, 177.4 mm for chestnut forest and 394.5 mm for paddy field.

Run-on water through the watershed is 1.4~4.1 times of rainfall annually, indicating that water resources wasn't utilized efficiently. Rainy season showed poor irrigation performance of 17.8%, mainly due to over-irrigation. After that, less water was distributed and the efficiency increases. The irrigation utilization efficiency was about 37.0% in normal year due to extensive irrigation, and about 54.7% in dry year. The irrigation efficiency was 41.8% on average at the small catchment. The temporal variation of decade interval will enhance the total quality of irrigation system performance. Such information is important to help government and irrigation managers make policy of water-saving and improve irrigation system performance.

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